The function of proteins which interact with the 5'-terminus of messenger RNA in eukaryotic cells will be examined. A protein will be purified from rabbit reticulocytes which has the activity of reversing inhibition by cap analogues and which is presumed to be the cap-binding protein. The covalent structure at he 5'-terminus of several chicken oviduct mRNAs, conalbumin, ovomucoid, and lysozyme, will be determined along with the initial nucleotide sequence. Factors which influence the stability of messenger RNA in vivo will be investigated in an organ culture from chick oviduct.